People interpret this in many ways of course. My experience of it is different. For me has to do with the progression of form, energy, and a balance of Yin and Yang.

At the basic level, an encounter (for Nage) starts from the square - grounded, and receptive and perceptive on all sides for the movement and energy to evolve in any direction. This is more of an embracing, Yin state.

As an intention begins to be manifested (by Uke), one responds in a triangular fashion - enter, pivot, whatever, it is all in dynamic alignment with a triangular orientation and movement, which is more of a responsive, Yang state.

As this process and interaction evolves toward completion, Kuzushi and then resolution come about through circular/spherical movements which resolve the intention and energy in a balanced, neutralizing fashion; not Yin, not Yang.

Square -> Triangle -> Circle (Sphere) -> Square…. etc.

This is reflective of a deeper, spiritual process of the evolution of consciousness, and transcendence of duality through the conscious resolution of tension. It is the tension between polarities, Yin and Yang, that creates duality. The circle is the continual release of this tension into Unity, thus, all Aikido evolves into it.